1. Field of the Invention
The present invention relates to a magnetic memory element, and a magnetic memory device and an information recording/reproducing apparatus using the same.
2. Description of the Related Art
The practical use of magnetic memories (MRAMs) has been studied and developed for many years. Among the magnetic memories, these days, a STT (Spin Transfer Torque) type has been vigorously studied and developed because of its advantages with respect to high speed and high integration. In order to obtain an increased output, the STT-MRAM generally includes a magnetic memory element with an insulating layer such as of MgO disposed between ferromagnetic layers, utilizing tunneling magneto-resistive effect (TMR).
When using the TMR element, however, it is required to decrease a junction resistance of the TMR element so as to reduce a critical current value of magnetization reversal due to spin injection, thereby avoiding deterioration of the insulating layer. In addition, if a resistance of the insulating layer increases with miniaturization of the element, not only a S/N ratio may decrease because of an increase in shot noise but also a bit transmission rate may decrease. When miniaturizing the element, moreover, since a shape error of the element has a great impact on magnetization characteristics and an output voltage value of the element, it is difficult to improve manufacturing yield. Thus, at present, the STT-MRAM has many problems about increasing the density of the element.
Regarding increasing the density of the element, for example, Japanese Unexamined Patent Application Publication No. 2004-146821 discloses a magnetic memory in which a fixed layer, a spin conductive layer, and a free layer are stacked within a carbon nanotube in the named order. Since the carbon nanotube is a minute tube with a diameter of the order of nm, as is well known, it can be used as a memory element to significantly increase the number of elements arranged within a unit area of the memory, thereby realizing a higher density.
However, since the above magnetic memory utilizes GMR effect similar to the TMR effect, it also has the foregoing problems. Particularly, since it is extremely difficult to accurately control the individual thicknesses of the three layers, i.e., the fixed layer, the spin conductive layer, and the free layer, within the minute carbon nanotube, the manufacturing yield becomes a major issue.